(PUP-3718) Make defined function handle qualified variable names

The defined function can answer if a variable is defined. There
were however no tests for qualified variable names. The implementation,
when given a qualified name simply tried to lookup the complete name
in its own scope which naturally failed.

This commit adds logic to check if a variable is qualified in the
Scope#exist? method, and then mimics the behavior of a variable
lookup by splitting the name, then looking up the class and its scope
and finally delegating the exist? call to that scope.

This is made somewhat complicated when testing due to the fact how
scopes are wired (to find other names scopes). The test that checks for
the existence of a top scope variable now sets this up as if the main
class (called '') had been evaluated.
(PUP-3718) Make it clear that exist?(varname) returns boolean

There was a code path that did not ensure that that the return
from exist?(varname) was a boolean. Now a test is added, and nil
result is always turned into a boolean.

This is primarily done to ensure against leaking values / object
that could be considered thruthy if this code changes. (It would
be harmeless to return nil/false or true as the code stands now,
but this commit makes it explicit that a boolean result is expected.
Merge pull request #3356 from hlindberg/PUP-3718_defined-function-misses-qualified-variables

(PUP-3718) Make defined function handle qualified variable names